1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the length of a buried conduit in advance when a cable is to be laid in the conduit, and also to a method and apparatus for diagnosing the inside of the conduit.
2. Description of the Related Art
As an infrastructure of a communication network in a high information society, heavy construction of user's optical lines is being advanced in order to make all up to general homes optical. Conventionally, in the case where optical cables are laid under the ground, such a method has been adopted that the cables are once inserted into a conduit, and thereafter are connected by welding optical fibers of the respective cables with each other. Working must be extremely simplified in order to realize huge construction of equipments to lead optical fibers from stations to respective homes, and in stead of such a method of connection by welding which takes a long time, such a method in which connectors are attached to opposite ends of every cable in advance so that connections are performed entirely through the connectors has been tried (1990, Paper of The Meeting of The Association of Electric/Information Related Societies: pp. 1-6).
In the case where a cable having connectors attached onto its opposite ends is to be laid in a long conduit, the cable cannot be used if it is shorter than the conduit. On the other hand, if it is longer than the conduit, it is necessary to uselessly dispose of the excess length of the cable in a manhole. It is therefore necessary to accurately measure the length of a conduit between manholes in advance. This technique has not yet been developed.